Temporal dynamics of stream water chemistry in the last free-flowing river draining the western Sierra Nevada, California

Temporal patterns of stream water chemistry were analyzed across the Cosumnes River Watershed (1989 km2) for water years 1999–2002 to quantify hydrobiogeochemical dynamics in the last free-flowing watershed draining the western Sierra Nevada, California. The Mediterranean climate of California produces a distinct annual hydrologic pattern with three seasons: baseflow, stormflow, and meltflow. The baseflow season (July–October) is dominated by groundwater chemistry that primarily originates from high elevations, and thus does not vary much across the basin. During the baseflow season discharge is negatively correlated to ionic concentration, and sediment and nutrients are generally below detection levels. The stormflow season (November–March) is separated into a flushing period (where discharge is positively correlated to river water conductivity) and a dilution period (where discharge is negatively correlated to conductivity). During average flow years, virtually the entire annual load of nutrients and sediment moves through the watershed during the stormflow season. Because stormflow hydrologically links the land with local waterways, the stormflow season shows the greatest variance among sites across the diverse landscape of the Cosumnes Watershed. Chemistry of the meltflow season (April–June) is dominated by dilute upland snowmelt, and there is little chemical variation across the watershed. Storm-scale analysis in water year 2002 revealed that progressive flushing occurs with each storm event and that source area dynamics play an important role in chemograph response. With 19 of the 20 major rivers in the western Sierra Nevada having dams, these data provide scientists and regulators with a valuable reference to address how impoundment affects water quality.